Synthesis gas production by steam reforming

ABSTRACT

Process for the preparation of hydrogen and carbon monoxide rich gas by steam reforming of hydrocarbon feedstock in presence of a steam reforming catalyst supported as thin film on the wall of a reactor, comprising steps of (a) optionally passing a process gas of hydrocarbon feedstock through a first reactor with a thin film of steam reforming catalyst supported on walls of the reactor in heat conducting relationship with a hot gas stream; (b) passing effluent from the first reactor to a subsequent tubular reactor being provided with a thin film of steam reforming catalyst and/or steam reforming catalyst pellets and being heated by burning of fuel, thereby obtaining a partially steam reformed gas effluent and a hot gas stream of flue gas; (c) passing the effluent from the second reactor to an autothermal reformer; and (d) withdrawing from the autothermal reformer a hot gas stream of product gas rich in hydrogen and carbon monoxide.

The present invention is directed to the production of synthesis gas bysteam reforming of a hydrocarbon feedstock in contact with catalyzedhardware.

The term catalyzed hardware is used for a catalyst system, where a layerof catalyst is fixed on a surface of another material, e.g. metallicsurfaces. The other material serves as the supporting structure givingstrength to the system. This allows to design catalyst shapes whichwould not have sufficient mechanical strength in itself. The systemherein consists of tubes on which a thin layer of reforming catalyst isplaced on the inner wall.

Alternative layouts may comprise of tubes with a catalyst layer on theoutside, plates with catalyst coating, or other suitable shapes.

Synthesis gas is produced from hydrocarbons by steam reforming by thereactions (1)–(3):C_(n)H_(m) +nH₂O→nCO+(n+m/2) H₂(−Δ₂₉₈ ^(o)<0)  (1)CO+H₂O⇄CO₂+H₂(−ΔH₂₉₈ ^(o)=41 kJ/mole)  (2)CH₄+H₂O⇄CO+3H₂(−ΔH₂₉₈ ^(o)=−206 kJ/mole)  (3)

A second method for production of synthesis gas is autothermal reforming(ATR). In autothermal reforming, combustion of hydrocarbon feed iscarried out with substoichiometric amounts of oxygen by flame reactionsin a burner combustion zone and, subsequently, steam reforming of thepartially combusted feedstock in a fixed bed of steam reformingcatalyst. The oxidant can be air, enriched air, or pure oxygen.

A third method for production of synthesis gas is the combination offirst passing the hydrocarbon feed through a fixed bed of reformingcatalyst and, subsequently, passing the partly reformed feed through anautothermal reformer. The fixed bed may comprise of a number of tubesplaced in a fired furnace. This combination is called two-step reformingor primary followed by secondary reforming and is particularly suitedfor production of synthesis gas for methanol and ammonia production. Bycontrolling the amount of reforming occurring in the fixed bed steamreformer before the ATR, a synthesis gas having the correctstoichiometry for methanol synthesis or a synthesis gas having thecorrect ratio of hydrogen to nitrogen for ammonia synthesis can beproduced.

State of the art steam reforming technology makes use of reformingcatalyst in the form of pellets of various sizes and shapes. Thecatalyst pellets are placed in fixed bed reactors (reformer tubes). Thereforming reaction is endothermic. In conventional reformers, thenecessary heat for the reaction is supplied from the environment outsidethe tubes usually by a combination of radiation and convection to theouter side of the reformer tube. The heat is transferred to the innerside of the tube by heat conduction through the tube wall and istransferred to the gas phase by convection. Finally, the heat istransferred from the gas phase to the catalyst pellet by convection. Thecatalyst temperature can be more than 100° C. lower than the inner tubewall temperature at the same axial position of the reformer tube.

It has been found that heat transport is more efficient when catalyzedhardware is used in the steam reforming process. The heat transport tothe catalyst occurs by conduction from the inner tube wall. This is amuch more efficient transport mechanism than the transport by convectionvia the gas phase. The result is that the temperatures of the inner tubewall and the catalyst are almost identical (the difference below 5° C.).Furthermore, the tube thickness can be reduced, see below, which makesthe temperature difference between the inner and outer side of thereformer tube smaller. It is hence possible to have both a highercatalyst temperature and a lower tube temperature, all other conditionsbeing the same when replacing the conventional reformer tubes withcatalyzed hardware tubes. A low outer tube wall temperature is desirablesince it prolongs the lifetime of the tube. A high catalyst temperatureis advantageous since the reaction rate increases with temperature andsince the equilibrium of reaction (3) is shifted to the right hand sideresulting in a better utilisation of the feed.

Pressure drop in the catalyzed reformer tube is much lower than in theconventional case for the same tube diameter. This enables the use ofreactors of non-traditional shapes e.g. tubes with small diameter andstill maintaining an acceptable pressure drop. Smaller tube diameterresults in an increased tube lifetime, tolerates higher temperatures andreduces the tube material consumption.

Finally, the catalyst amount is reduced when using catalyzed hardwarereformer tubes compared to the conventional reformer with a fixed bed ofreforming catalyst.

The small amount of catalyst dictates the use of a feedstock free ofcatalyst poisons. This can e.g. be obtained by sending the feedstockthrough a prereformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a plant producing syn gas.

FIG. 2 shows the temperature profile imposed on the reactor wallmeasured by the movable thermocouple.

FIG. 1 shows an example of a plant producing syngas. Feed 2 ispreheated, desulphurized in unit 4, mixed with process steam 6, andfurther heated before entering an adiabatic prereformer 8. The effluentstream from prereformer 8 is further heated in a heat exchanger coilarranged in flue gas channel 12 and send to the tubular reformer 14,where conversion of methane to hydrogen, carbon monoxide, and carbondioxide occurs. The effluent gas is passed to autothermal reformer 16,wherein combustion is performed with oxidant stream 18. The processingof effluent gas 20 downstream from the autothermal reformer depends onthe intended use of the product.

Catalyzed hardware can be used in two of the units shown in FIG. 1:

1. In the preheater coil 10 for heating the prereformer effluent gasbefore entering the tubular reformer 14.

2. In the tubular reformer 14.

This invention provides process for the preparation of hydrogen andcarbon monoxide rich gas by steam reforming of a hydrocarbon feedstockin presence of a steam reforming catalyst supported as thin film on thewall of a reactor, comprising steps of

(a) optionally passing a process gas of hydrocarbon feedstock through afirst reactor with a thin film of steam reforming catalyst supported onwalls of the reactor in heat conducting relationship with a hot gasstream;

(b) passing effluent from the first reactor to a subsequent tubularreactor being provided with a thin film of steam reforming catalystand/or steam reforming catalyst pellets and being heated by burning offuel, thereby obtaining a partially steam reformed gas effluent and ahot gas stream of flue gas;(c) passing the effluent from the second reactor to an autothermalreformer; and(d) withdrawing from the autothermal reformer a hot gas stream ofproduct gas rich in hydrogen and carbon monoxide.

EXAMPLE 1

A catalyzed hardware reformer reactor has been tested. The test unitconsists of a system for providing the feeds to the reactor, the reactoritself, and equipment for posttreatment and analysis of the effluent gasfrom the reactor.

The reactor consists of a ¼″ tube of length 1050 mm which is, in themiddle 500 mm, coated on the inner wall with RKNR nickel steam reformingcatalyst. The catalyst has the same composition as the RKNR pelletshapedsteam reforming catalyst available from Haldor Topsoe A/S. The thicknessof the catalyst layer is 0.31 mm. The catalyzed reactor tube is placedin a casing made of solid metal, which has a hole closing tightly aroundthe catalyzed tube. A number of milled grooves, in which thermocouplesare placed, is made along the hole. One of the thermocouples is movableso that a wall temperature profile of the catalyzed tube can beobtained. Additionally, thermocouples are placed in the gas channel inthe catalyzed tube measuring the gas temperature at the inlet to and theoutlet from the catalyzed zone. The reactor with casing is placed in anelectrically heated oven, in which the temperature can be controlledseperately in 6 different zones.

The feed steams consist of hydrogen, methane, carbondioxide, and steam.The feed streams are mixed and preheated before entering the reactor.After the reactor, the effluent gas stream is cooled down, the condensedwater is separated from the gas, and the gas composition is measured bya gaschromatograph.

Two sets of conditions were tested. One set at lower temperature tosimulate use of catalyzed hardware in a preheater coil (test No. 1), andone set at higher temperature to simulate a tubular reformer (test No.2). The conditions are shown in Table 1. The pressure was in both cases28 bar g. The temperature profile imposed on the reactor wall measuredby the movable thermocouple is shown in FIG. 2.

TABLE 1 Gas temperature Gas temperature Carbon- at inlet of at outlet ofHydrogen Methane dioxide Steam catalyzed zone catalyzed zone flow rateflow rate flow rate flow rate Test No. ° C. ° C. Nl/h Nl/h Nl/h Nl/h 1605 633 62.0 310.0 16.1 781.4 2 679 795 240.5 152.0 63.1 425.0The measured effluent gas composition is shown in Table 2. The gascomposition is on dry basis.

TABLE 2 Effluent Gas Composition on Dry Basis Carbon- Carbon- Hydrogenmonoxide dioxide Methane Test No. mole % mole % mole % mole % 1 49.62.48 11.4 36.5 2 67.8 10.8 9.80 11.7

The effluent gas is in both cases in equilibrium with respect to thereforming reaction at the outlet gas temperature within experimentaluncertainty. This demonstrates that a conversion similar to a fixed bedreactor can be obtained in a catalyzed hardware reactor.

1. A process for the preparation of hydrogen and carbon monoxide richgas by steam reforming of a hydrocarbon feedstock in the presence of asteam reforming catalyst supported as thin film on a wall of a reactor,comprising the steps of: (a) passing a process gas of hydrocarbonfeedstock through a first reactor (10) having an inner wall and an outerwall, wherein a thin film of steam reforming catalyst is supported onthe inner wall of the reactor (10), wherein the first reactor (10) is apreheater coil, and wherein the reactor (10) is in a heat conductingrelationship with a hot gas stream of flue gas; (b) passing effluentfrom the first reactor (10) to a subsequent tubular reactor (14)comprising at least one reformer tube having an inner wall and an outerwall and being provided with a steam reforming catalyst, wherein thesteam reforming catalyst is a thin film of steam reforming catalystsupported on the inner wall of the at least one reformer tube and/orsteam reforming catalyst pellets and wherein the at least one reformertube is heated by burning of fuel, thereby obtaining a partially steamreformed gas effluent and the hot gas stream of flue gas; (c) passingthe effluent from the second reactor (14) to an autothermal reformer(16); and (d) withdrawing from the autothermal reformer (16) a hot gasstream of product gas (20) rich in hydrogen and carbon monoxide.
 2. Theprocess of claim 1, wherein the steam reforming catalyst comprisesnickel and/or noble metals.
 3. The process of claim 1, wherein theprocess gas of step a) is effluent from an adiabatic prereformer.
 4. Theprocess of claim 1, wherein the tubular reactor (14) comprises aplurality of reformer tubes having a thin film of steam reformingcatalyst supported on the inner wall of the reformer tubes.